What uses port 139
Before Windows 2000, SMB file sharing ran over NetBIOS on port 139 (with name service on UDP 137 and datagrams on UDP 138). Modern Windows uses SMB directly on port 445, but keeps 139 for compatibility with old clients. On current networks you can usually disable NetBIOS over TCP/IP entirely.
Should port 139 be open to the internet?
Keep private. Never expose this port to the internet. Keep it on localhost, a private network or behind a VPN or SSH tunnel.
Check what is listening on port 139
sudo ss -ltnp 'sport = :139'sudo lsof -nP -iTCP:139 -sTCP:LISTENGet-NetTCPConnection -LocalPort 139 -State Listen
netstat -ano | findstr :139No output means nothing is listening. If a service should be, check that it is running and which address it binds to: 127.0.0.1 only accepts local connections, 0.0.0.0 or :: accepts them on every interface.
Fix “address already in use” on port 139
Another process already holds the port. Find it with the commands above, then stop that service properly, or end the process:
sudo kill $(sudo lsof -t -iTCP:139 -sTCP:LISTEN)Stop-Process -Id (Get-NetTCPConnection -LocalPort 139 -State Listen).OwningProcessAllow port 139 from trusted networks only
The examples allow 139 only from 10.0.0.0/8; replace it with your own private network, office IP or VPN range.
sudo ufw allow from 10.0.0.0/8 to any port 139 proto tcp
# Close it again:
sudo ufw delete allow from 10.0.0.0/8 to any port 139 proto tcpsudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.0.0.0/8" port port="139" protocol="tcp" accept'
sudo firewall-cmd --reloadNew-NetFirewallRule -DisplayName "NetBIOS Session 139/TCP" -Direction Inbound -Protocol TCP -LocalPort 139 -Action Allow -RemoteAddress 10.0.0.0/8On AWS, Google Cloud and Azure, also allow the port in the security group or network firewall rules; host firewalls and cloud firewalls are separate.
Test port 139 from another machine
nc -zv example.com 139Test-NetConnection example.com -Port 139Related ports
- Port 445 (SMB): used by SMB for Windows file and printer sharing.
- Port 135 (Microsoft RPC): the Microsoft RPC endpoint mapper, used by many Windows services.
- Port 137 (NetBIOS Name Service): used by NetBIOS name service for legacy Windows name resolution on local networks.
More file sharing ports
20 (FTP data) , 21 (FTP) , 69 (TFTP) , 873 (rsync) , 990 (FTPS) , 2049 (NFS) . See all ports in the port lookup.
Sources
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Frequently asked questions
What is port 139 used for?
Port 139 is used by NetBIOS session service, the legacy transport for Windows file sharing. Before Windows 2000, SMB file sharing ran over NetBIOS on port 139 (with name service on UDP 137 and datagrams on UDP 138). Modern Windows uses SMB directly on port 445, but keeps 139 for compatibility with old clients. On current networks you can usually disable NetBIOS over TCP/IP entirely.
Is it safe to open port 139?
Never expose this port to the internet. Keep it on localhost, a private network or behind a VPN or SSH tunnel. Allow it only from the networks that need it, as in the firewall examples on this page.
How do I check if port 139 is open?
On the server, list listening sockets with "sudo ss -ltnp 'sport = :139'" on Linux or "sudo lsof -nP -i :139" on macOS. From another machine, run "nc -zv host 139" or, on Windows, "Test-NetConnection host -Port 139". Connection refused means nothing is listening; a timeout means a firewall is dropping the traffic.